If the planet has a massive H\(_2\)  atmosphere, thermochemical reactions in the deep atmosphere recycle O, C, N, S species into H\(_2\) O, CH\(_4\) , NH\(_3\) , and H\(_2\) S [1]}, [2]}, [3]}, [4]}. H\(_2\) O can form a cloud and the above-cloud H\(_2\) O may be partially depleted as a result [5]}, [6]}, [7]}. Recent calculations have shown that the photodissociation of NH\(_3\)  in the presence of CH\(_4\)  leads to the formation of HCN and N2, and that CO and CO\(_2\)  are produced by the photodissociation of CH\(_4\)  together with H\(_2\) O [7]}. The photodissociation of H\(_2\) S leads to the formation of elemental sulfur haze [9]}, [10]}, but the haze would likely be close to the cloud deck and would not mute transmission spectral features [7]}. These photochemical products are transported to the deep atmosphere and recycled back to CH\(_4\) , NH\(_3\) , and H\(_2\) S. An exception is that planets with super-solar atmospheric metallicity and appreciable internal heat may have additional CO, CO2, and N2 transported from the deep troposphere and incomplete recycling to NH3 [12]}, [13]}, [7]}.
